Hey Saskia - Feathers has it down pretty well.
And do look into high metabolism. It's a problem I have: I'm 6'1" and it took me ten years to go from 145Lbs to 155... and then I lost that 10Lbs in about two months from slacking on my protein intake!
I eat every two hours, lots of healthy calories and protein from various sources, and hopefully this will get some meat on my body. Diets are individual things, but you could look into trying something like what I'm doing out.
The thing with the proteins is that if you don't work out, it will just turn to straight fat, instead of muscle, so it depends how you want the extra weight to look on you.
If your stomache is growling, immediately feed it! That noise/feeling is the body going into a catabolic state (opposite of anabolic, or muscle growth) and is starting to break down your own muscle tissue as supplement since there's no food coming in for nutrients. Sounds disgusting, but it's just part of the body's defences, and also it's way of telling you to eat!
Here is a link to a free calorie-counter, that tells you roughly how many calories your body should be burning per day (add some extra if you do have high-metabolism)
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Calorie Calculator
- as it says below the counter, if you want to gain weight, eat 500-1000 more calories/day than the number that is given you.
Omega-3's are awesome, and chicken is great for protein. Beef is good, but as we're learning, too much beef causes some to stay in the system's intestinal tract for the rest of your life and is thought to lead to colon cancer.
Free-range, organic beef is the way to go: most, if not all regular processed beef has bovine growth hormones injected into it (it's not mandatory for beef companies to share this information, at least not over here), hormones which have been linked to breast cancer.
There's a lot of stuff to remember concerning a person's health! But once you get everything straight and figured out it comes naturally.